Dielectric combiner technologies have been used primarily in the construction of Heads Up Displays (HUD). Combiners are well known in military applications, specifically for their ability in the cockpit of fighter jets to display information to pilots across the windscreen without obstructing the pilots, view of the outside world. These combiners are unique in there ability to selectively reflect light within a narrow range of wavelengths and thereby reducing glare while remaining optically clear.
The photocatalytic dielectric combiner in this application functions as a UV surface enhancer. The invention decomposes contaminants and harmful bacteria that contact the UV enhanced surface by the photocatalytic oxidation of titanium oxide (TiO.sub.2). Since the Fujishima-Honda effect-involving photoelectro-chemical reactions of a semiconducting TiO.sub.2 electrode was reported in 1972, attention has been focused on TiO.sub.2 as a practical photocatalyst. It has been applied to a variety of environmental problems in addition to water and air purification. In the last few years, research and applications of this photocatalyst have been increasing. This increase may be because it has been proven that only a small amount of UV radiation from various types of lamps, such as fluorescent lamps, is necessary for the photocatalytic decomposition of nearly all contaminants in indoor and outdoor environments. Additionally, it has been demonstrated that the TiO.sub.2 photocatalyst is effective for deodorization, sterilization of bacteria, and decomposition of organic contaminants.
In general, photocatalytic reactions are caused by the irradiation of light on semiconductors, particularly where TiO.sub.2 is the photocatalytic material. When the photon energy is greater than or equal to the band gap energy of TiO.sub.2, i.e., E=3.2 ev or .lambda..ltoreq.400 nm, an electron (e-) is promoted from the valence band into the conduction band, leaving a hole behind. Some of the electrons which have been excited into the conduction band and some of the holes in the valence band recombine and dissipate the input energy as heat. However, a number of holes can diffuse to the surface of the TiO.sub.2 and react with .sup.- OH absorbed on the surface. This reaction forms .sup.- OH radicals, which can decompose the organic compounds existing on the surface into CO.sub.2 and H.sub.2 O because the potential energy of the .sup.- OH radical is greater than the bonding energy of almost all organic compounds.
Currently, TiO.sub.2 films are deposited on various substrates using the Sol-Gel method. The film comprises one or more layers of photoreactive gelatin which has (or have) been subsequently developed by wet chemical processing as disclosed in "Applications of Photocatalytic Reactions Caused by TiO.sub.2 Film to Improve the Maintenance Factor of Lighting Systems" by H. Honda, A. Ishizaki (1), R. Soma (2), K. Hashimoto, and A. Fujishima (3) in the Winter 1997 issue of JOURNAL of the Illuminating Engineering Society. Honda, et al. discloses a substrate which is dipped into a titanium alkoxide solution, a TPT monomer, or a polymer chelated with a glycol polymer. There may be variations in the mixture as far as what is used but the processing manner is essentially the same, i.e., the substrate is pulled out at a rate which determines the coating thickness. The coated substrate is then heated at about 600.degree. C. to form the crystalline anatase phase.
It usually desirable to construct the film with fringes that are parallel to the surface of the gelatin. However, design constraints, such as optically recorded noise patterns, may prevent this construction. In these cases, the fringes intersect the surface and form a slant fringe pattern which produces extraneous diffraction images. These images would make such films unacceptable for windows, viewing monitors, and decorative applications.
Another limitation with the sol-gel method is mechanical abrasive damage. Such damage is due to the film being an organic material which is extremely susceptible. A further problem with the sol-gel method is the tendency of the layers of the film to delaminate both from the substrate and from adjacent layers. The delamination is due to the differing coefficients of expansion between the various layers of the film. The substrate is subjected to varying thermal conditions, so the layers expand and shrink at varying ratios resulting in delamination between the layers and substrate. It would be desirable to develop a photocatalytic titanium dioxide (TiO.sub.2) film which is hard, durable, and abrasion resistant, and does not delaminate or distort surface images. A film can also be deposited on a variety of substrate materials such as plastics, metals, glass, and composites, and may be used in various applications such as window, decorative, medical, food handling and institutional applications to provide a self-cleaning, self-sanitizing and self-deodorizing surface. In addition the UV enhanced dielectric combiner can be placed on an organic leveling polymer to provide a viable alternative to electroplating. Articles such as costume jewelry, television antennas, luggage latches, trophies, lighting fixtures, watches, plumbing fixtures, door hardware, automotive wheels, washing machine and dryer dials, and the like are commonly plated. These surfaces are often in contact with bacteria and, therefore, could benefit from a self-disinfecting, self-sanitizing, hard, durable coating. Consumers typically love the shiny bright chrome and gold-tone finishes that add appeal to many items they purchase. The decorative industry produces these finishes on various kinds of substrates, including but not limited to plastic, zinc, aluminum, and brass, by electroplating. A typical faucet or door hardware manufacturer may electroplate about 90 to 95 percent of all the products produced, and in some companies in the decorative industry the percentage could be 100% of total products produced.
The interest in replacing the electroplating process in decorative applications, and finding alternative methods of coating soft substrates, such as brass, zinc, aluminum, and plastics, is not new. The primary goals are to achieve substrate containment, leveling, desired hardness, surface brilliance, and scratch- and corrosion-resistant surfaces so that maintenance is minimized.